Are you saying Bret or Undertaker were rivaling Hogan and Warrior in popularity in 1991? The former, I get. The latter, no.To me it appears Vince didn't trust Undertaker, but I get it with his gimmick at the time. In 1991, he was rivaling Hogan and Ultimate Warrior in popularity. He was never seen as someone who could be their heir apparent in terms of being the champion and carrying the company.
He trusted Bret more ironically than Taker, despite Taker being put into the main attraction spot as Andre.
I agree that Vince was reticent to have a gimmick as the champion. It's the same issue that arises with all invulnerable gimmicks: how do you get the belt off them? And, also, no matter how 'cool' it is to see someone who is invulnerable, fan engagement with that diminishes with time because how can a fan really relate to that? So it works as an attraction, but not as the focal point. It's why we saw Undertaker's gimmick develop over the years with him being 'humanised' at various points.